I do have SU active, and I can not tell you why your Windows rebooted. May be because the new installed program needed it to install completely. Now, I use SU only to warn me of any outdated software in case I missed any of the new updates. I rather go to the program site and download/install the update myself. That way I can make sure nothing else is installing with it. If you want to deactivate the outdated program to install automatically:
Open avast! > Settings > Tools > Customize > Uncheck " Automatically download updates if possible " see image